Mouse livers were perfused at 22 °C with an oxygenated salts medium containing [5−3H]cytidine and [carboxyl-14C]inulin. The cellular uptake of cytidine was determined from the3H content of liver samples less that present in the extracellular (inulin) space of the samples. Time courses of cytidine uptake were biphasic with initial phases which were approximately linear for 15 s and had time zero values that approximated the extracellular space. Rates of cytidine uptake derived from the initial phase of uptake evidently represented rates of membrane transport because (i) initial rates were saturable, and (ii) cytidine uptake was blocked by the nucleoside transport inhibitor, nitrobenzylthioinosine (NBMPR). Treatment of mice with the 5′-monophosphate of NBMPR (> 0.2 mg/kg, injected i.p.) 30–40 min prior to the perfusion also blocked cytidine entry into livers. An apparent half-saturation constant of about 10−3 Mand a maximum rate of about 1 μmol∙g−1∙min−1were estimated for the NBMPR-sensitive transport of cytidine into mouse liver ce